Not for the first time lately – indeed, not for the first time at this address – I have found myself relishing the sensation of being caught by a dramatic sucker punch: a piece of work which seems, predictably and uninterestingly, to be heading irrevocably in one direction, but which then swerves into altogether more fascinating territory. This time the perpetrator is Gary Owen’s latest play, which visits the National Theatre’s mid-sized receiving space in a production that premièred last spring at Sherman Cymru in Cardiff.

Sophie Melville’s unsettlingly upfront Effie begins by challenging the audience in what she claims (probably rightly) are our middle-class assumptions about her. Clad in hoodie and faded Dayglo leopard-print leggings, Effie prowls the stage recounting her lairy, drink- and drug-fuelled exploits in the less salubrious inner-city areas of Cardiff (of which Splott is one).

Those of us who recognise the classical allusion in the title begin to wonder where the analogy is with the daughter whom Agamemnon sacrificed in order to give the Greek army fair winds as they sailed off to besiege Troy. Eventually matters swim into focus: Effie, having refrained from accusing the married ex-soldier who knocked her up on a one-night stand, loses boyfriend, home and such prospects as she had in her own mind. So far, so sharply written, keenly performed and (by Rachel O’Riordan) astutely directed, but still no more than just another gritty urban monologue.

This, just as familiarity begins to breed contempt, is where Owen slews his vehicle into the oncoming traffic. As Effie’s unborn baby begins to act up in its second trimester, the final third of the 80-minute piece becomes an account of her serial sufferings due to health service funding cuts. The greater her physical and emotional agony, the more incandescent the political indictment. Suddenly the play reveals its purpose: to testify how countless ordinary folk are being sacrificed to grant fair economic winds to a dubious ideological enterprise. When the initial challenge to us is reprised, it serves to clarify that Effie’s own restraint by not pressing for compensation has saved at least some resources for us, and to ask the burning question: what will happen when all such patience is exhausted?

Written for the Financial Times.